1. Field of the Invention
The present invention relates to the generation of seismic waves and more particularly to the generation of seismic shear waves downhole.
2. Related Prior Art
The seismic exploration industry has immediate need for a downhole seismic source capable of generating strong vertical seismic shear waves at frequencies from 10 to 600 Hz. Such a source is not now available. Presently, there are at least three current attempts to develop such sources. All of these sources clamp to the borehole casing and shake the casing vertically with a vibratory sweep.
In one attempt, a development firm has been working on a voice-coil actuated source, which uses a very small internal reaction mass and a very heavy external housing. This apparatus must operate continuously at resonance throughout a vibratory sweep and uses a variable pressure gas volume and a feedback control system to make this resonance possible. It appears that this firm has not developed an effective sidewall clamping mechanism and have not yet tested its sources in a well.
A second development firm, in cooperation with several large corporations, is promoting the development of a pneumatic vibratory source which, like the first source, depends upon continuous resonance for effective operation. So far, the prototype of this concept has operated with a band width that is too limited and an output signal that is too weak for applications which the general industry is considering.
One company has built and extensively tested a prototype source based upon their U.S. Pat. No. 4,702,343, issued on Oct. 27, 1987. The concept in this patent is basically a downhole application of VIBROSEIS. According to released data, this source generates a very strong output signal in the frequency range from 10 to 600 Hz. This concept uses an external reaction mass, which so far has restricted applications to well bores in which no liquids are present. There appears to be plans to further develop the SV version of this source.
Other U.S. patents that are indicative of the state of the art are listed below.
U.S. Pat. No. 4,923,030, titled "Device for Generating Acoustic Waves by Causing a Falling Mass to Strike a Target Element Coupled With the Walls of a Well", issued to Patric Mevnier relates to a device for generating acoustic waves in a well by causing a mass falling inside a body to strike a target element and to transmit impacts to the surrounding geological formations by anchoring the body in the well by retractable shoes. A tractive force exerted on the support cable from the surface, after anchorage of the body, causes a support element to move upwards which takes the target element and the mass with it. When the mass is hydraulically locked in its set position, the cable is slackened so that the target element and the support element can come back to the bottom position. In this position, the support element presses on a pusher element which actuates hydraulic locking and releases the mass.
U.S. Pat. No. 4,648,478, titled "Device for Generating Sound Pulses Inside a Well by Percussion", issued to Pascal Dedole, et al. relates to a device for generating sound pulses in a well by a mobile mass striking against a target element integral with an elongate body anchored in a well by retractable shoes actuated by hydraulic cylinders. The mobile mass which slides in a first chamber in the body is moved away by a piston moving in a second chamber of the body and intermittently locked in the moved away position by a valve. The piston, the valve and also the hydraulic cylinders acting on the anchorage shoes are actuated by a hydraulic system which may be remote controlled from the surface.
U.S. Pat. No. 4,991,685, titled "Downhole Seismic Source", issued to Tom P. Airhart, relates to a downhole seismic generator that can be located in various positions in the well including being located in engagement with the bottom of the well bore. The generator includes a large striker mass driven by compressed gas into engagement with an anvil having one end located in engagement with the medium in which the seismic signals are to be set up. A retractor is provided to return the mass to ready position so that a series of seismic signals can be provided as desire. An optional baffle that is connected to the generator prevents waves from traveling through the well bore which would otherwise diminish or confuse the seismic shock wave initially generated.
U.S. Pat. No. 4,796,723, titled "System for Transmitting Energy Through an Apparatus Usable for Subsoil Prospection Lowered Inside a Well of Drill Hole", issued to Jean Laurent, et al. relates to a system for transmitting energy to an apparatus, such as a pulsed or vibratory seismic source, lowered inside a well or drill hole. It includes a structure anchorable in a well, a mobile device whose linear displacement with respect to the anchorable structure causes the creation of energy and a device for accumulating the energy created, a rigid column connected to the mobile device and a driver disposed outside the well for moving the rigid column linearly. The mobile device and the device for accumulating energy may be formed by a weight and a device for raising this latter or else by a hydraulic system including a linear pump.
U.S. Pat No. 5,031,717, titled "Advanced Downhole Periodic Seismic Generator", issued to Harry C. Hardee, et al., relates to a downhole periodic seismic generator system for transmitting variable frequency, predominantly shear-wave vibration into earth strata surrounding a borehole. The system comprises a unitary housing operably connected to a well head by support and electrical cabling and contains a clamping apparatus for selectively clamping the housing to the wall of the borehole. The system further comprises a variable speed pneumatic oscillator and a self-contained pneumatic reservoir for producing a frequency-swept output over a discrete frequency range.
U.S. Pat. No. 5,031,719, titled "Seismic Sonde", issued to Roy Baria. et al. relates to an instrument for sensing seismic waves and determining their direction of propagation. This instrument is operable a considerable depth in a borehole. It comprises a short, stubby sensor module with a device to clamp it to the borehole wall and includes three accelerometers. The instrument is connected by a flexible umbilical cored to a primary module suspended by a cable. The down hole electronics and the power supply for the clamp are provided in the primary module. The sensor module has a fundamental resonant frequency about 1 kHz, well above the frequency of the seismic waves it is subjected to, so that the signals from the accelerometers accurately represent those waves.
U.S. Pat. No. 4,805,725, and U. S. Pat. No. 4,702,343 both titled "Nondestructive Downhole Seismic Vibrator Source and Processes of Utilizing the Vibrator to Obtain Information About Geologic Formations", issued to Bjorn N.P. Paulsson, relate to a nondestructive downhole seismic source capable of generating Sv-Waves, Sh-Waves, and P-Waves alone or in combination to determine information about a surrounding geologic formation. The patent also includes processes of performing cross well tomography and reverse vertical seismic profiling. The patent also includes a means and process to carry out in hole seismic logging operations.
U.S. Pat. No. 4,715,470, titled "Downhole Electromagnetic Seismic Source", issued to Bjorn N.P. Paulsson, relates to a downhole seismic source capable of generating seismic forces in excess of 1000 newtons. The source produces seismic waves for seismic applications, particularly cross borehole measurements and vertical seismic profiling. Coupled with motion sensing devices, the source also performs as a seismic logging tool. The source is comprised of an outer housing, a device for clamping the source securely to the wellbore, and a linear electromagnetic actuator which utilizes permanent magnetic material having remanent magnetic field exceeding about 0.9 T such as a rare earth permanent magnetic. The source has a diameter of about 12.5 cm to fit within a typical wellbore and the actuator has a length of about 1 meter.